The present invention relates to a sputtering target which contains boron (B) and is produced via the melting and casting method, and in particular relates to a target effective for use in a magnetic head, a magnetoresistive element (MRAM) or the like, in which gaseous impurities are few, compositional segregation is minimal, and there is no problem in mechanical properties such as cracks.
As the next-generation high-speed storage element, a magnetoresistive element (MRAM) is being developed, and a magnetic material containing boron (B) is being used as the material for use in layers configuring the MRAM. For example, known are compositions made from elements such as Co, Fe and Ni, and boron, even specifically Co—B or Co—Fe—B, or compositions in which Al, Cu, Mn, Ni and the like are added thereto.
Generally speaking, the magnetic layers configuring the MRAM are prepared by sputtering a target having a composition made of Co, Fe, Ni and the like and boron.
Since this kind of magnetic sputtering target comprises B as a main component, particularly if the composition ratio of B is increased and exceeds 10%, extremely fragile Co3B, Co2B, and CoB compound phases are formed. Consequently, the ingot becomes cracked or fractured, and it is difficult to obtain a sputtering target from the ingot.
When the ingot is prepared via slow cooling in order to prevent the foregoing problem, it is possible to inhibit the generation of cracks and fractures, but there is a problem in that compositional segregation will increase. Thus, targets have been obtained by subjecting powders to sintering and molding, but since the surface area of the powders is large and the adsorbed gas has trouble escaping, the amount of gaseous impurities, particularly oxygen, will increase. Consequently, there is a problem in that the characteristics of the film that is obtained via sputtering will be unstable.
Here, there are various types of sputtering devices, but a magnetron sputtering device comprising a DC power supply is widely used in the deposition of the foregoing magnetic film in light of its high productivity. The sputtering method is to place a target as the negative electrode opposite to a substrate as the positive electrode, and generate an electric field by applying a high voltage between the substrate and the target in an inert gas atmosphere.
Here, the inert gas is ionized to form plasma made of electrons and positive ions. When the positive ions in the plasma collide with the surface of the target (negative electrode), the atoms configuring the target are sputtered, and the sputtered atoms adhere to the opposing substrate surface and thereby form a film. This stands by the principle that the material configuring the target is deposited on the substrate as a result of this sequence of processes.
As described above, when using a sputtering target that is produced by sintering raw material powders, there is a significant drawback in that large amounts of oxygen are contained in comparison to the melted and casted material and that the density is low. The existence of large amounts of gas components such as oxygen generates particles during the sputtering of the target, and causes the magnetic properties of the magnetic film to be unstable. Accordingly, a method of adding B to a magnetic raw material for deoxidation has been proposed (refer to Patent Document 1).
Nevertheless, in the foregoing case, deoxidation is performed by adding B to an alloy mainly based on Co and melting the product, rapid solidification treatment and powderization are performed thereto, and sintering is further performed to the obtained powder in order to obtain a powder sintered target. In Patent Document 1, boron is added in an amount of 10% or less in order to eliminate oxygen from the raw metal such as Co, and this is to aim for deoxidation by adding B in a midway process. However, since the powder sintering method is ultimately adopted, as shown in the Examples and Comparative Examples of Patent Document 1, the results are inferior in terms of oxygen content and density in comparison to the melting and casting method, and it cannot be used for producing a target in which the composition of B needs to be controlled.
Normally, the oxygen content achieved by the powder sintering method is 150 wtppm or more. In order to further reduce the oxygen content, processes of the high costs need to be devised, and this is undesirable from the perspective of actual production.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2001-107226